The term "hybrid devices" refers to (among other things) devices which include a ceramic substrate on which are mounted various components, e.g., semiconductor integrated circuits, capacitors, inductors, and the like. Electrical connections among the various components are by means of conductive films of material, generally in the form of narrow strips, which extend between the components along the surfaces of the substrate.
While not critical, the films have a thickness generally in the range of between 5 and 30 micrometers, and are referred to as "thick" films. In the fabrication of such films, they are generally first deposited, through a patterned screen, onto the workpiece in the form of an ink. The ink is dried and then fired to remove the various organic ink binders and to fuse together the particles of the film and adhere it to the ceramic substrate.
One extensively used thick film conductor comprises a sintered mixture of silver and palladium, the composition being, depending upon the particular device application, generally in the range of 65 to 85%, by weight, silver, balance palladium. Such silver-palladium (Ag-Pd) based films are quite stable, both physically and chemically, adhere well to various ceramics, and form excellent surfaces onto which various device components can be readily and reliably bonded, particularly by soldering. A problem with such Ag-Pd films, however, is that owing to the relatively high content of palladium thereof, they are rather expensive and possess high electrical resistivity.
For purposes of cost savings, it is often the practice to use such Ag-Pd films only on "critical" areas of the substrate, e.g., where components are to be soldered, and to use less expensive silver based conductive films as extensions of the Ag-Pd films. Such pure silver films cannot fully replace the Ag-Pd films because they are not adequately stable. For example, the essentially pure silver films tend to migrate along the surface of the substrate (particularly in humid environments), hence cannot be used where extremely close spacings between films are required, and the silver films are easily leached away by commonly used solders thus leading to unreliable joints.
A further problem with the use of pure silver films is that they tend to form alloys with the Ag-Pd films where joints between the two are required. During such alloying, silver from one of the films (as described hereinafter) migrates into the other film with the possible result that one of the films becomes so thin that a discontinuity or break occurs between the two films where they first make contact. Heretofore, this undesirable result has been generally avoided by insuring that there is an adequate thickness of both films where they contact one another. Thus, even with the occurrence of the alloying and migration of silver between the films, each film remains integral and without discontinuities therein.
A growing problem, however, is that the trend is to make hybrid devices ever smaller and of greater component density. This greatly reduces the flexibility of design of the conductive paths, and conditions often arise (described hereinafter) where it is extremely difficult to provide an adequate thickness of the two films where they contact one another. Thus, discontinuities often develop. While it is possible, of course, to make greater use of the Ag-Pd films, this is undesirable due to high cost and high electrical resistivity. Accordingly, a need exists for improved silver based conductors which are significantly less expensive than the Ag-Pd conductors, but which possess low electrical resistivity and form reliable joints or connections therewith even under adverse, reduced film thickness conditions.